It is well-known in the packaging industry to employ laminated packaging material of a single-use nature for packing and transporting liquid foods. Normally, such laminated packaging materials are built up from a configurationally rigid but foldable core layer consisting of, for example, paper or paperboard in order to achieve good mechanical configurational stability. Liquid-tight coatings of plastic are applied on both sides of the core layer and effectively protect the core layer of liquid-absorbing fibre from penetration by moisture. These outer layers normally consist of a thermoplastic, preferably polyethylene, which moreover impart to the packaging material superior thermosealing properties, whereby the packaging material may be converted into finished packages with the desired geometric configuration.
However, laminated packaging material consisting solely of paper or paperboard and liquid-tight plastic lacks tightness properties vis-6-vis gases, in particular oxygen gas. This is a major drawback in the packing of many foods whose shelf-life deteriorates dramatically when in contact with oxygen gas, such as for example fruitjuices. In order to supplement the packaging material with a barrier against gases, especially oxygen gas, it is known in the art to apply a layer possessing superior oxygen gas tightness properties, for example aluminum foil or polyvinyl alcohol, on that side of the core layer which is intended to face in towards the inside of the package.
In comparison with aluminum foil, polyvinyl alcohol enjoys many desirable properties, with the result that it is preferred as barrier material in many contexts.
In comparison with aluminium foil, polyvinyl alcohol enjoys many desirable properties, with the result that it is preferred as barrier material in many contexts. Among these, mention might be made of the polyvinyl alcohol's superior strength properties, compatibility with foods and economic value, together with its excellent oxygen gas barrier properties. Moreover, it has been considered to be expedient, in certain cases from the point of view the environment and recycling, to replace aluminum foil as the gas barrier material in food packages.
Like many other conceivable barrier or adhesive polymers such as, for example, ethylene vinyl alcohol, starch, starch derivative, carboxy methyl cellulose and other cellulose derivatives or mixtures thereof, polyvinyl alcohol is suitably applied by means of a coating process, i.e. in the form of a dispersion or aqueous solution which, on application, is spread out to a thin, uniform layer on the substrate and thereafter dried. We have found that one drawback in this process however is that an aqueous polymer dispersion or polymer solution of, for example, polyvinyl alcohol, optionally with an addition of EAA, which is applied on a core layer of paper or paperboard penetrates into the liquid-absorbing fibres of the core layer. In connection with the removal of water for drying and possibly for curing the applied barrier layer, the core layer is also subjected to elevated temperatures for drying, and as a result the risk of undesirable crack formation in the paperboard or paper layer, respectively, increases as a result of the result of the moisture content which is difficult to adjust, and the drying which takes place in this layer.
Swedish Patent No. 440519 proposed including a thickening agent such as alginate to reduce penetration of water into the board. The use of PVOH as a barrier material applied over a polymer layer preventing crack formation and smoothing the board surface was disclosed in WO97/13639.
One drawback is that the polyvinyl alcohol is moisture sensitive and rapidly loses its barrier properties when it is exposed to a damp environment. This inconvenience was previously obviated according to WO97/22536 by combining the polyvinyl alcohol with one or more per se known food-approved polymers, for example ethylene acrylic acid copolymer (EAA) or styrene butadiene copolymer. These advantageously form, in combination with polyvinyl alcohol, a coherent, well-integrated layer possessing superior gas barrier properties, in particular oxygen gas barrier properties, at the same time as the desired superior gas barrier properties of the polyvinyl alcohol are retained even in a damp environment.
WO97/22536 disclosed that polyvinylalcohol mixed with EAA-ethylene copolymer or the like material could be dispersion coated onto a paperboard previously coated with a polymer and thereafter could be dried and cured at temperatures of up to 170° C. to form a laminated packaging material with a very good barrier property and with good adhesion between the barrier layer and the substrate layer.
Without being restricted to any particular theory, it is suggested that the improved oxygen and water barrier properties results from an esterification reaction between the PVOH and the EAA at the increased curing temperature, whereby the PVOH is crosslinked by hydrophobic EAA polymer chains, which thereby are built into the structure of the PVOH.
Another drawback in the employment of, for example, polyvinyl alcohol as barrier layer instead of aluminum foil is that, on storage of light-sensitive foods, it is necessary in many cases also to incorporate into the packaging material a light barrier of some type. Granted, a core layer of paper or paperboard does not (to the naked eye) allow the passage of any light, but light in invisible wavelength ranges nevertheless penetrates through from the outside of a packaging container to the packed food product and may have a negative effect on it from the point of view of shelf-life. The employment of aluminum foil in the packaging material enjoys that advantage that the aluminum foil in itself constitutes a good barrier against both gases and against light. On the other hand, polyvinyl alcohol is as good as completely transparent even in mixtures with a hydrophobic polymer such as ethylene acrylic acid copolymer or styrene butadiene copolymer. The admixture of conventional light barriers, such as carbon black and titanium dioxide into any of the plastic layers included in the laminated packaging material according to WO97/22536 is per se possible, but would entail an aesthetically unattractive appearance in the package.
Yet a further drawback inherent in the laminated packaging material including barrier layers of, for example, polyvinyl alcohol possibly together with another polymer is that this packaging material cannot be produced employing the same production equipment as in the production of packaging material using aluminum foil as the barrier layer, which involves capital investment costs for new production equipment.